Over the past several decades, incidence rates of non-Hodgkin's lymphoma (NHL) have increased dramatically throughout the world among both men and women. In spite of the considerable public health significance, little is known about the etiology of NHL. Hair coloring products have been suggested as risk factors for NHL because some of the constituents have been found to be mutagenic and/or carcinogenic. However, the results from epidemiologic studies have been inconsistent. Several epidemiologic studies have suggested that the risk of NHL from hair coloring products varies by disease subtype and by the type of hair dyes. However, individual epidemiologic studies have been limited in their ability to consider the impact of risk factors on NHL by both exposure and disease subtypes because of small sample sizes. In addition, because hair dye formula contents have been changed around 1980, it is crucial to examine the relationship between hair coloring products and NHL risk by time period of use, but none of the previous epidemiological studies have examined this relationship by time period of use. Considering millions of people using hair coloring products each year and the uncertainty surrounding the relationship between hair coloring product use and risk of NHL, there exists an urgent need to assess the impact of hair dyes on NHL using a larger sample size. The International Consortium of Investigators Working on Lymphoma Epidemiologic Studies (InterLymph) was established to facilitate the cooperation between large, epidemiologic studies of NHL, providing us with an opportunity to explore the relationships between hair coloring products and NHL. Therefore, we propose to conduct a pooled analysis of data from several case-control studies that are a part of InterLymph. The primary aim of this pooled analysis is to determine if the risk of NHL is associated with hair coloring products, and whether this risk varies by disease subtype, type of hair coloring products, and time period of use. This InterLymph-based pooled analysis will include three population-based case-control studies conducted in the United States and one multi-center case-control study conducted in Europe, resulting in a total of 4,893 histologically-confirmed, incident cases and 6,589 controls, from six SEER sites and six European countries. The increased sample size that will result from the collaboration of several large, epidemiologic studies will enable us to have sufficient statistical power to examine the strength and consistency of the associations between hair coloring products and NHL.